1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, to such a semiconductor device provided with a sense amplifier for amplifying a potential difference between bit lines.
2. Description of Prior Art
The semiconductor device such as a DRAM (Dynamic Random Access Memory) is designed to amplify a potential difference appearing between a pair of bit lines by using a sense amplifier. A generally-used sense amplifier is constituted by N channel-type MOS transistors cross-coupled with each other and P channel-type MOS transistors cross-coupled with each other, and when an operation voltage is supplied to a common source of each transistor, the potential difference between the paired bit lines is amplified. Moreover, semiconductor devices disclosed in JP-A No. 2008-16145, JP-A No. 2008-159188 and JP-A No. 2010-102808 are designed to overdrive the common source in the initial stage of the sensing operation in order to improve a sensing speed.
The common source of the sense amplifier is equalized to the same electric potential during a precharge period. When an active command is issued, an operation voltage is supplied to the common source, thereby bringing to a state where a reading operation or a writing operation can be carried out through the sense amplifier. This state continues until a precharge command has been issued.
In this case, however, when an operation voltage of a restore level has been continuously supplied to the common source during all the period from the issuance of the active command to the issuance of the precharge command, a problem arises in which a consumption current increases because of a leak current flowing through the sense amplifier. In order to solve this problem, JP-A No. 2008-123609 has proposed a method for equalizing the sense amplifier during one portion of the period from the issuance of the active command to the issuance of the precharge command.
However, in an attempt to equalize the sense amplifier during the active period, switching is required so as to separate the sense amplifier and the bit line from each other in the period. Moreover, since during the period in which the sense amplifier is being equalized, the bit line is brought into a floating state, information held in the memory cell might be lost when the equalizing period is long.